This invention relates to a plant to precipitate suspended matter out of water.
Industrial waste water, landfill leachates, biologically treated waste water and even surface run-off water contain pollutants which must be separated out of the water. These pollutants are either bound to turbid elements or are dissolved in the water. In the first case they are destabilized by the introduction of suitable additives such as ferric(III) salts or aluminium salts. In the second case precipitation with, for example, carbonates or sulphides or adsorption with activated carbon must take place prior to the destabilization. Due to the destabilizing, particles are formed which can be agglomerated together and which can in suitable reactors, such as stirring tanks or cylinder stirrers or in conduits with turbulent flow, grow into flocculates which can be separated out. They can be separated from the water by sedimentation, flotation or by filtration.
With sedimentation and flotation fine flocculates remain in the water which must then be removed by the subsequent filter stage. If filtration is used to remove the flocculates the filter can in some cases block early before that the required amount of filtrate for backwashing has been obtained. The aim has therefore been to combine filtration with one of the two other processes in such a manner that filtration is the last treatment stage.
Even though the advantages of the combination of the two process stages are obvious and have also been discussed at length in publications most plants to be found in practice only consist of a sedimentation stage or of a flotation stage i.e. the filtration stage is missing. The reason is that the filtration stage is regarded as being too complicated, too costly and too prone to break-downs.
The term "filtration" is understood in water treatment terminology to be deep filtration in filter tanks using granular material. The filters are filled with gravel, pumice or anthracite and the direction of flow is from top to bottom. During backwashing, water is directed through the filter in the opposite direction from bottom to top so that the granular material fluidizes and is greatly agitated. The dirt which has settled on the granular material is washed off particularly well in this manner.
With other construction configurations, filtration takes place from bottom to top--that is in the same direction as the backwashing. In this case heavier dirt particles must also be washed through the filter layer as they arrive at the bottom of the filter layer with the untreated water and remain there at first. Other configurations use granular form material which is lighter than water. The upwardly directed filtration can, in this manner, be advantageously combined with a downward backwashing process. In order to prevent the foam material from flowing out of the plant the top and the bottom of the filter area are each closed off with sieves.
Backwashing is a multistage process, in particular in the case of upward backwashing, as for example is the case with gravel filters. The water stored above the filter must at first be let flow out, the filter bed must be broken up by introducing air and special pumps must be available and switched on in order to be able to obtain the high backwash velocity required. The backwash water must be drained off and the filter must then be filled up again prior to the renewed starting up of the filtration process. In all, a large number of measurements must be taken and a large number of regulatory steps must be taken and adhered to. By backwashing the filter filled with granular foam material in a downward fashion, the breaking up of the filter with air is no longer necessary and this halves the cost of the backwashing process. The most outstanding characteristic is nevertheless the strong pumps required for the backwashing process.
A plant for the sedimentation and filtration of water using a flotation plant with a plate separator situated below it is described in DE 35 40 508 A1. The direction of flow of the water to be treated is from bottom to top. Some of the filter material in the flotation plant is removed discontinuously from the lower filtration zone and is cleaned externally in a backwash tank with a rinsing liquid after which the filter material is again introduced to the top of the bed of filter material.
Disadvantages of this plant are that the backwashing process takes place outside the plant, that the backwashing process is carried out using freshwater or using a special rinsing liquid and that only a fraction of the total filter material can be cleaned in the backwash tank. The danger also exists that if the pump fails or if the direction of flow changes the filter material falls downwards and can flow out through the outflow pipe in an uncontrolled manner. The renewed starting up of the plant which has been blocked in this fashion is a lengthy operation.